1. Field of the Invention
The present invention relates generally to an engine starting system for restarting an engine which has once stopped automatically under engine idle conditions in response to an engine restart request, and in particular to a start control technique used for restarting an engine when an engine restart request is given in an engine stopping period which begins at a point of fuel supply interruption and ends at a point of complete engine stop.
2. Description of the Related Art
There exist conventionally known engine control systems devised for reducing fuel consumption and carbon dioxide (CO2) emissions, for instance. One example of such engine control systems is an idle stop system which causes an engine to automatically stop during engine idle operation and automatically restart when an engine restart request, such as a driver's operation for starting, occurs.
A generally known method of restarting a once stopped engine is to start the engine by cranking an engine output shaft by means of an external driving device, such as a starter motor. Using this conventional approach employing a starter motor in an idle stop system is impractical, however. Since the idle stop system stops and restarts the engine each time the engine is brought to idle, the starter motor must restart the engine a significantly larger number of times compared to a case where the engine is not stopped at idle. Therefore, the starter motor, if employed in an idle stop system, must have considerably high durability, which results in an unwanted cost increase.
Under these circumstances, techniques for restarting the engine without using the starter motor have been developed in recent years. According to an engine restarting method disclosed in Japanese Laid open Utility Model Publication No. 1985-128975 and Japanese Laid-open Patent Publication No. 2003-517134, for example, fuel is injected into a cylinder which is on an expansion stroke under engine stop conditions and a mixture thus produced in the cylinder is ignited and burnt to provide a starting torque for restarting the engine. In this kind of technique for restarting the engine without using the starter motor or the like, the starting torque produced at engine restart varies depending on where a piston in the cylinder on the expansion stroke stops at engine stop. Therefore, the conventional idle stop system controls the engine such that the piston in the cylinder on the expansion stroke stops at a position within a specified range of crank angle suitable for restarting the engine at engine stop by using a braking device or by regulating valve opening as shown in the aforementioned Publications.
On the other hand, Japanese Laid-open Patent Publication No. 2003-515052 discloses another engine starting technique which makes it possible to obtain a greater starting torque than obtained by the techniques of the aforementioned prior art Publications. According to this alternative engine starting technique, fuel is injected into a cylinder which is on an expansion stroke under engine stop conditions and a mixture thus produced in the cylinder is ignited and burnt to rotate a crankshaft slightly in a reverse direction and thereby compress the mixture in the cylinder on the expansion stroke. The compressed mixture in the cylinder is ignited and burnt again to eventually restart the engine.
Generally, even after fuel supply is interrupted, an engine rotates several times by inertia before it completely stops. It may commonly be understood that, if an engine restart request is given during this engine stopping period, the once stopped engine could be restarted by injecting fuel into a cylinder which is currently on an intake or expansion stroke and igniting a mixture produced in the cylinder in the same way as in an ordinary engine start sequence.
Immediately before complete engine stop when engine speed is extremely low, however, rotational inertial force of the engine is extremely small. Even when ordinary start-up control operation is performed as described above under such conditions, an adequate starting torque may not be obtained, resulting in a failure to restart the engine. To ensure successful engine restart, the engine control systems of the prior art Publications wait for a cylinder to reach a crank angle falling within the aforementioned specified range before restarting the engine when an engine restart request, such as depression of an accelerator pedal, is given.
It is recognized from the foregoing discussion that the conventional engine control systems have a problem that these engine control systems must wait for the engine to completely stop before restarting the engine if an engine restart request is given in the engine stopping period by the driver or else, and this causes some delay in restarting the engine from the point of the engine restart request.